Monday's pre-dawn shooting on the bus in Kandahar province's Zhari district left four dead and another 18 people wounded, Ayubi said. He said international forces took 12 of the wounded to a military hospital.

NATO and Afghan authorities declined to identify the international forces involved by nationality, although numerous Afghans said they were American. More than 80,000 US troops are in Afghanistan, many of them deployed to volatile southern provinces including Kandahar.

A passenger interviewed at Kandahar hospital, Rozi Mohammad, said they had just left the Kandahar terminal when the bus pulled over to allow an American convoy to pass. Shooting broke out as the third or fourth American vehicle went by, he said, with gunfire coming from the direction of the convoy.

“They just suddenly opened fire, I don't know why. We had been stopped and after that I don't know what happened,” said Mohammad, his left eye was swollen shut and his beard and clothing matted with blood. Doctors said he had suffered a head injury but did not yet know how serious it was.

NATO said the bus had approached a slow-moving patrol from behind at a high speed and ignored flares and other signals to slow down.

“Perceiving a threat when the vehicle approached once more at an increased rate of speed, the patrol attempted to warn off the vehicle with hand signals prior to firing upon it. Once engaged, the vehicle then stopped,” NATO said in a statement e-mailed to reporters.

Accounts of such confrontations often vary widely, and NATO said it was carrying out further investigations together with Afghan authorities.

Within hours of the shooting, scores of Afghans had blocked the main highway out of Kandahar city with burning tires, chanting “Death to America,” and calling for the downfall of Afghan President Hamid Karzai, himself a Kandahar native.

“The Americans are constantly killing our civilians and the government is not demanding an explanation,” said resident Mohammad Razaq. “We demand justice from the Karzai government and the punishment of those soldiers responsible.” Karzai issued a statement condemning the attack and expressing condolences to the victims.

“This shooting involving a civilian bus violates NATO's commitment to safeguard civilian life,” Karzai said.